Scientists invert time in a quantum computer

Time goes in one direction: forward. The little boys become older men but not the opposite; Tea cups break but never come together spontaneously. This cruel and immutable property of the universe, called "arrow of time," is fundamentally a consequence of the second law of thermodynamics, according to which systems will always tend to become more disordered over time. But recently, American and Russian researchers have slightly bent this arrow, at least for subatomic particles.

In the new study, published Tuesday, March 12 in the journal Scientific Reports, researchers manipulated the arrow of time with the help of a very small quantum computer composed of two quantum particles, known as qubits , allowing to perform calculations. [Twisted Physics: 7 Mind-Blowing Findings]

At the subatomic scale, where the strange rules of quantum mechanics rule, physicists describe the state of systems through a mathematical construction called wave function. This function is an expression of all the possible states in which the system could be located – even in the case of a particle, of all possible locations in which it could be located – and of the probability that the system is located in any of these states at any given time. . Generally, over time, wave functions develop; The possible location of a particle can be further away if you wait an hour than if you wait 5 minutes.

To cancel the spread of the wave function is to try to put the spilled milk back into the bottle. But that's exactly what the researchers accomplished in this new experience.

"There is virtually no chance that this will happen alone," Valerii Vinokur, principal investigator at the Argonne National Laboratory in Illinois, told Live Science. "It's like saying, if you give a typewriter to a monkey and that he spends a lot of time, he could write Shakespeare." In other words, it is technically possible but so unlikely that it is also impossible.

How did scientists make impossible impossible? By carefully controlling the experience.

"You really need a lot of control to replenish all the pieces of a cup of tea," said Stephen Bartlett, professor of physics at the University of Sydney, at Live Science. Bartlett was not involved in the study. "You have to have a lot of control over the system to be able to do it … and a quantum computer is something that allows us to have a lot of control over a simulated quantum system."

The researchers used a quantum computer to simulate a single particle, its wave function spreading over time like a ripple in a pond. Then they wrote an algorithm in the quantum computer that reversed the temporal evolution of each component of the wave function, essentially bringing this ripple back into the particle that created it. They accomplished this feat without increasing entropy or creating disorder elsewhere in the universe, apparently defying the arrow of time.

Does this mean that researchers have created a time machine? Did they violate the laws of physics? The answer is no to both questions. The second law of thermodynamics says that the order of the universe must decrease with time, but not that it can never remain the same in very special cases. And this experience was small enough, short enough, and controlled enough that the universe did not gain or lose energy.

"It's very complicated and complicated to send waves on a pond" once they've been created, Vinokur said, "but we saw that it was possible in the quantum world, in a very simple case. " In other words, it was possible when they used the control that gave them the quantum computer to cancel the effect of time.

After running the program, the system returned to its original state 85% of the time. However, when a third qubit was introduced, the experiment only succeeded 50% of the time. The researchers said the complexity of the system probably increased too much with the third qubit, making it more difficult for the quantum computer to keep control of all aspects of the system. Without this control, entropy can not be controlled and the time reversal is therefore imperfect. Still, they are targeting larger systems and larger quantum computers for their next steps, Vinokur told Live Science.

"This work is a valuable contribution to the fundamentals of physics," Live Science professor James Whitfield, a physics professor at Dartmouth College in New Hampshire, who did not participate in the study, told Live Science. "This reminds us that not all applications of quantum computing need to be application-oriented to be interesting."

"That's exactly why we're building quantum computers," said Bartlett. "It's a demonstration that quantum computers can enable us to simulate things that should not happen in the real world."